The long-read assembly of Apareiodon sp., a neotropical fish with a ZZ/ZW sex chromosome system

The landscape of transposable element distribution in the genome of Neotropical fish Apareiodon sp. (Characiformes: Parodontidae)

Transposable elements (TEs) are widely present in eukaryotic genomes, where they can contribute to genome size and functional modifications. As new genomes are sequenced and annotated, more studies can be conducted regarding TE content, distribution, and genome evolution. TEs are extensively diversified in fish genomes resulting in an important role in genome and chromosome evolution. However, curated TE libraries are still scarce in non-model organisms, making it difficult to evaluate TE's impact on genomic modifications thoroughly. Here, we aimed to obtain a curated TE library from the neotropical fish Apareiodon sp. genome. The prospection and curation of the TE library resulted in 244 families from 18 superfamilies of DNA transposons and retrotransposons, which comprise about 10% of the genome, with most insertions fitting in one or a few families. A greater diversity of retrotransposon families is present, especially for Ty3 superfamily. Despite the greater number of retrotransposon families, DNA transposons are the most abundant in the genome, with 37% of all TE insertions belonging to the Tc1-Mariner superfamily. Complete TE copies were observed for almost all superfamilies, with most of the sequences on the Tc1-Mariner group. DNA transposons and SINEs presented older insertions in the genome, followed by LINEs and LTR retrotransposons. TE genome density is highest in the cs25 scaffold, and enriched for Helitron elements. With these data, allied to previous studies on chromosome evolution, we suggest that cs25 bears the W chromosome specific region of the Apareiodon sp. genome, with the presence of significant amount of Helitron insertions.

Repetitive DNAs and differentiation of the ZZ/ZW sex chromosome system in the combtail fish Belontia hasselti (Perciformes: Osphronemidae)

BACKGROUND

Java combtail fish Belontia hasselti (Cuvier, 1831), a member of the Osphronemidae family, inhabits lakes and rivers throughout Southeast Asia and Sri Lanka. Previous cytogenetic research revealed it possesses a diploid chromosome number of 48 chromosomes with a female-heterogametic ZZ/ZW sex chromosome system, where the W chromosome is distinguishable as the only metacentric element in the complement. Female-heterogametic sex chromosome systems seem to be otherwise surprisingly rare in the highly diverse order Perciformes and, therefore, B. hasselti provides an important comparative model to evolutionary studies in this teleost lineage. To examine the level of sex chromosome differentiation in B. hasselti and the contribution of repetitive DNAs to this process we combined bioinformatic analyses with chromosomal mapping of selected repetitive DNA classes, and comparative genomic hybridization.

RESULTS

By providing the first satellitome study in Perciformes, we herein identified 13 satellite DNA monomers in B. hasselti, suggesting a very low diversity of satDNA in this fish species. Using fluorescence in situ hybridization, we revealed detectable clusters on chromosomes only for four satellite DNA monomers. Together with the two mapped microsatellite motifs, the repeats primarily accumulated on autosomes, with no distinct clusters located on the sex chromosomes. Comparative genomic hybridization showed no region with accumulated female-specific or enriched repeats on the W chromosome. Telomeric repeats terminated all chromosomes, and no additional interstitial sites were detected.

CONCLUSION

These data collectively indicate a low degree of sex chromosome differentiation in B. hasselti despite their considerable heteromorphy. Possible mechanisms that may underlie this pattern are discussed.

Characterization of hAT DNA transposon superfamily in the genome of Neotropical fish Apareiodon sp

DNA transposons are diverse in fish genomes and have been described to generate genomic evolutionary novelties. hAT transposable element data are scarce in Teleostei genomes, making it challenging to conduct comparative genomic studies to understand their neutrality or function. This study aimed to perform a genomic and molecular characterization of hAT copies to assess the diversity of these elements and associate changes in these sequences to genomic and karyotypic novelties in Apareiodon sp. The data revealed that hAT TEs are highly abundant in the Apareiodon sp. genome, with few possibly autonomous copies. Highly conserved sequences with likely functional transposases were observed in nine hAT elements. A great diversity of hAT subgroups was observed, especially from Ac, Charlie, Blackjack, Tip100, hAT6, and hAT5, and a similar wave of hAT genomic invasion was identified in the genome for these six groups of hAT sequences. The data also revealed a distinct number of microsatellites within degenerated hAT copies. hAT sites were demonstrated to be dispersed in the Apareiodon sp. chromosomes and not involved in W chromosome-specific region differentiation. In conclusion, the genomic analysis revealed a great diversity of hAT elements, possible autonomous copies, and differentiation of degenerated transposable elements into tandem sequences.